This invention relates to an AC generator.
There is known a three-phase AC generator as shown in FIGS. 1 and 2 which is broadly used for automobiles. The AC generator of this type has a voltage regulator incorporated therein, which is disclosed e.g. in Japanese Publication of Utility Model Application No. 44214/1981.
FIG. 1 is a cross sectional view showing the construction of the prior-art AC generator which essentially comprises: a rotor 11 comprising a pole core 111 of carbon steel, a field (magnetic field) coil 112, a yoke 113 and a slip ring 114; a stator 12 having a three phase winding and comprising a stator (armature) core 121 and a stator coil 122; a front case 13 for supporting the rotor 11 and the stator 12; a rear case 14; a diode 21 which is supported in the rear case 14; a voltage regulator 3; a fan 15; a pulley 16; a brush holder 17; a brush 18; and a bearing 19.
FIG. 2 is an electric circuit diagram used for the AC generator 1 of FIG. 1, wherein contained in the AC generator 1 are the field coil 112 and the stator coil 122 which are connected so as to provide a three-phase star connection. A full-wave rectifier comprising six diodes 21 is indicated by reference numeral 2.
The voltage regulator 3 for regulating the output voltage of the AC generator 1 to a predetermined value comprises a diode 31 for absorbing the surge current across the field col 112; a darlington transistor 32 for switching the field coil 112; a resistor 33 connected to the base of this darlington transistor 32; a controlling transistor 34 for controlling the switching function of the darlington transistor 32; a Zener diode 35 which detects the output voltage of the AC generator 1 and, when this output voltage is larger than a given level, supplies the current from the AC generator to the base of the transistor 34; and resistors 36 and 37 connected in series for providing a voltage dividing circuit for the output voltage of the AC generator, wherein the voltage dividing point is indicated by reference numeral 38.
The circuit in FIG. 2 additionally includes an ignition switch 4, a battery 5, a load 7 such as a heater or head lights, etc. and a switch 6 to activate the load 7.
In this circuit of FIG. 2, when the key switch 4 is closed to start the engine of the automobile, currents are passed from the battery 5 through the key switch 4 and the resistor 33 to the base of the darlington transistor 32. Thus, the darlington transistor 32 is closed or made conducting and the field current flows from the battery 5 through this darlington transistor 32 to the field coil 112 to excite the pole core 111.
In this state, the engine is started and the rotor 11 is rotated through the pulley 16, whereby an AC output voltage is induced in the stator coil 122 in response to the rotating speed of the rotor 11. The output voltage is rectified by the full-wave rectifier 2. When this rectified output voltage is smaller than a predetermined value, the voltage at the voltage dividing point 38 of the voltage dividing circuit of the resistors 36 and 37 is still low so that the Zener diode 35 is kept non-conducting, wherein the output voltage of the AC generator 1 increases as the rotating speed of the rotor 11 increases.
Thereafter, when the output volage becomes larger than a predetermined value, the voltage at the voltage dividing point 38 of the above-mentioned voltage dividing circuit reaches the point where the Zener diode 35 is made conducting. The current through this Zener diode 35 flows to the base of the controlling transistor 34, so that the transistor 34 is made conducting. At this time, the darlington transistor 32 is made non-conducting, and it causes the field current to the field coil 112 to be turned off thereby decreasing the output voltage of the AC generator 1. When this output voltage is decreased to a predetermined value, the Zener diode 35 and the transistor 34 are made non-conducting again with the darlington transistor 32 conducting. Thus, the field current flows through the field coil 112 to excite the pole core 111, so that the output voltage of the AC generator 1 increases again. The above-described operation is repeated and the output voltage of the AC generator 1 is controlled to a predetermined value. The battery 5 is charged up to a predetermined voltage by this controlled voltage.
However, in such a prior-art AC generator, even when the output voltage is smaller than a predetermined value, the battery is charged with the field current which is always supplied from the battery 5 to the field coil 112. When the load 7 is operated by closing the switch 6 in the state mentioned above, the AC generator 1 has to generate the extra electric power to be consumed by the load 7 in addition to the electric power to be supplied to the field coil 112 to charge the battery. Thus, a higher output generator is required for the AC generator 1, which will become large and heavy, and will increase the production cost.
A primary object of the present invention is to eliminate the above described drawbacks by providing a compact and light-weight AC generator in which the load of the field coil in the generator for power generation can be reduced, and sufficient output voltage required can be obtained using a generator the necessary output power of which is lower than the conventional one. In addition, the adjustment of the generator is easily done with a simplified voltage regulating circuit.